Hi Przemek,
First I must admit, I didn't try the following at all. I don't know what is an L-stage form either. But I think the following might provide some direction.
You can use the FieldFactory class to get a Field object, using the form name and field ID.
Then you can use the getDisplayInstance method of Field object to have a DisplayInstanceList object. There might be several labels for a field if there are more views than one. You can use the method getProperty (of DisplayInstanceList) with a specific view ID and appropriate tag to get the label. For data fields, you can use Constants.AR_DPROP_LABEL tag, for example. For other types (like buttons, attachment fields etc.) you need to find out which tags work.
I hope this helps.
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~soba